The technique called “jet grouting” consists in the consolidation of soil portions by means of injections at very high pressure of cementitious grouts through nozzles arranged at the bottom of a string of tubular rods. Jet grouting systems have been developed over time in order to meet all the needs of the field, and are distinguished for the number of the fluids which are used (only cementitious grout, cementitious grout plus air, cementitious grout plus air and water), and for the operative parameters which change the diameters of the consolidated soil from a few ten centimeters to above 3 m. The methods to carry out the treatment can be classified as: “continuous” or “stepwise”.
In the continuous method, the injection mainly occurs by combining the rotational and translational movement of the rods; rotational rate of the rods, ascent rate, flow rates and pressures of the consolidating fluids, are related to the diameter of the column to be created, the resistances required for the consolidated soil, and the jet grouting type selected (single-, double-, or triple-fluid type).
The stepwise withdrawal injection method distinguishes itself from the continuous method since the injection of consolidating grout occurs by alternating steps of only rotating the rod without pulling it out for a preset period of time, to withdrawal steps, performed in order to locate the nozzles to the upper bench. Therefore, the columnar treatment results to be composed of many stepwise consolidated soil “arches”. The limitations of this system relate to the part of the instruments on board of the machine, which is more complex, and gives a higher variability in keeping the treatment operative parameters set. On the other hand, the rotary head can be moved in a quicker manner compared to the continuous method. However, the “restart” limitations, related to the head stroke described before, remain unaltered.
In order to perform consolidating operations in the context of using jet grouting techniques, depths ranging between 15 and 50 m are generally reached. The vertical stroke available to the rotary head (defined as “rotary” in the field, since it delivers the drilling torque necessary for the rod to rotate during drilling) is generally not sufficient, since the pieces of equipment of a more widespread size typically have a mast with a length ranging between 4-7 m. Some special jet grouting equipment can have strokes up to 15-18 m, but this involves problems of weight, transportation costs, they require large spaces and well-leveled soils, and assembling times. Furthermore, the drilling machine, which is no more self-erecting, requires an auxiliary crane for all the handling steps of the mast.
Therefore, in order to reach the design depths, it is necessary to add rods to the drill string. However, this is a time-consuming and costly practice, since the operation to add and remove rods involves the risk of introducing soils into the duct, and of consequently causing the obstruction of the same duct.
In some cases, to increase the treatment depths, use is made of mast extensions which allow housing a string of rods much longer than that which the mast on which the rotary slides can house. In this case, the rods are passing “through” within the rotary, which drags them via locking means.
In this case, the drilling and treatment operations are performed in more “restarts” of the rod to reach the designated depths. When the rotary head has reached the topmost point of the guide mast, the so-called “restart” of the rod is performed: the drill string is locked and temporarily overhung by means of a clamp assembly at the mast base. Then, the rotary head performs a downward return stroke, then starting again with a new ascent and injection step (jet grouting).
In some of the main applicative fields of this technology, it is required to create a curtain wall, formed by the combination of partially overlapped jet grouting elements (diaphragm walls for surface excavations, impermeable shields for dams, impermeabilization of joints between adjacent buried panels, weirs). In these cases, the implementation of a series of consolidated soil columns with a horizontal section which is not circular, but instead elongated, typically in the aligning direction of the curtain wall or weir, in order to have a higher level of certainty of an impermeable junction thereof, can be cost-effective. Furthermore, the elongated shape decreases the number of elements needed to complete the diaphragm wall, and consequently the joints needed, the overlapped part of adjacent “columns”, with time and cost saving due to the less consolidating material to be injected into the hole.
EP 1 862 596 A1 discloses a system to implement consolidated soil columns with elongated shape composed of a rotary head (or “rotary”) which drives upon rotating a string of rods terminating at the ends thereof with an injection head (or “monitor”) provided with nozzles for the ejection of the consolidating grouts into the soil. A device, including projecting tabs secured to the rotating part of the rotation head and facing a proximity sensor integral to the rotation head fixed part, allows activating the different treatment modes, by modulating the adjustment of the drilling machine hydraulic circuit, to increment or slow down the rotational rate as a function of the head instantaneous angular position. The horizontal size of the consolidated soil element is as a function of the specific energy of the jet, and consequently (while keeping pressure and flow rate constant) of the time of exposure to the jet. In this case, the time of exposure is given by the rotational rate with which the jet encounters the soil body to be consolidated, beside by the ascent rate. Consequently, the rotational rate is inversely proportional to the specific energy inputted into the soil. High specific energy values allow implementing a higher diameter of treatment.
In EP 1 862 596 A1, the angular position of the nozzles is derived by detecting the rotary angular position. This system loses in precision where angular sliding movements between rotary and rod are generated. Such problem occurs when, due to the need to increase the treatment depth, use is made of mast extensions allowing housing a string of rods much longer than that which the mast on which the rotary slides can admit. In this case, the rods are passing “through” the internal part of the rotary, and no more directly secured thereto. Therefore, the transfer of the drilling movements from the rotary to the rods occurs by interposition of a third member, called through clamp or clamp jet, which receives the rotation from the rotary and transfers it to the rods by means of a clamping system based on wedges which transfer by friction these rotational components to the rod (which usually have a perfectly cylindrical and smooth outer profile).
In some cases, for example under the action of an insufficient clamping by the clamp on the rod, or a loosening of the same clamping due to impacts and vibrations, or due to sudden overloads typical of this type of underground operations, which can instantaneously halt the tool, thus creating a significant inertia on the motion transmission system, or still due to the progressive wear of the toothing located on the wedge surface, in direct contact with the rod, in all these cases, a sliding between rod and clamp wedges takes place, consequently between rod and rotary. It shall be apparent that this drawback does not involve penalizations in the case of cylindrical columns, while with elongated members, an assessment error of the nozzle position, which is instead integral to the rod, generates a column which is horizontally elongated to an undesired direction; this involves an insufficient copenetration and junction of adjacent panels, with consequent loss of impermeability of the underground structure. In those case where such defect is noticed, can be repaired by performing additional drilling operations and curtain wall treatments. Instead, where this defect were not noticed, the structural integrity of the structure to be implemented could be compromised, with a far greater impact on costs.